Electrical and mechanical equipment used in telecommunications and transportation applications typically require cooling fans to cool their associated electrical components. Such fans commonly are exposed to challenging physical environments, including salt spray, water, dust, impulse shock loadings, and vibration. Exposure to such challenging physical environments increases the failure rate and reduces the service lifetime of such cooling fan motors.
Some applications protect the electronics by means of a conformal coating--a thin layer of protective material which is applied by aerosol spray or by dipping the components in liquid conformal material. When the material hardens, a relatively thin vapor-proof barrier protects the components. However, conventional conformal coatings are a poor choice for use in electric motors because of the broad thermal cycling range of their component parts. As the motor runs, the components generate significant heat due to the flow of electric current and friction effects. The relatively thin layer of conformal coating is typically unable to expand and contract with the heating and cooling (after operation stops) of the underlying components, thus compromising the structural integrity of the conformal shell.
Moreover, conformal coatings provide little, if any, protection from the adverse effects of mechanical shock and vibration. One example of such effects is a wire clothes hanger which is broken by repeated flexing of the same spot. The hanger breaks easily because the metal structure has been substantially damaged and weakened at the spot of repeated flexing, a process known as work hardening. Similarly, exposure to mechanical shock and vibration causes work hardening of the various component parts of motors. After sufficient length of time, catastrophic failure of the physical structure of such component parts occurs.
Some methods, e.g., Marshall et al., U.S. Pat. No. 4,922,604, have attempted to provide environmental protection by embedding components of a motor, during the motor assembly process, in protective encapsulating material that is much thicker than known conformal coatings. These approaches have the advantage of providing the desired environmental protection across a broad thermal operating range. However, such methods encapsulate components as part of the motor assembly process, increasing the cost and complexity of assembly. For example, the encapsulating process described in Marshall et al. requires that an encapsulated stator assembly subsequently be machine bored, for instance by diamond lapping, to allow for the later insertion of a motor rotor. Another drawback of encapsulating motor components as part of the motor assembly process is the uneconomic encapsulation of assemblies and subassemblies which may later fail to pass one or more post-assembly quality control tests and be rejected.